I'm taking a text from MySQL DB using a Perl AGI Script, and then playing the TTS audio using Swift Application from the same AGI Script.
It plays TTS ok, but I'm having problems when the text includes "acuted" letters. For example: á é í ó ú
It seems to be reading it as % (percentage) symbol, but I don't hear it very clear.

It sounds like you are sending swift text encoded in a format it wasn't expecting. You need to set the encoding format for the engine. On the command line, this is done with -e followed by one of "utf-8", "utf-16", "iso8859-1" or "iso8859-15".